Java methods and exception handling

Separate conversion errors from valid numeric values.

Java

class Main {
    static void showNumber(String text) {
        try {
            int value = Integer.parseInt(text);
            System.out.println(value * 2);
        } catch (NumberFormatException error) {
            System.out.println("Invalid integer");
        }
    }
    public static void main(String[] args) {
        showNumber("12");
        showNumber("abc");
    }
}

Output

24
Invalid integer

Explanation

Integer.parseInt converts decimal text within the int range. Invalid syntax or an out-of-range value throws NumberFormatException.

A try/catch handles the specific conversion failure so the program can continue. Validation rules such as an allowed age range should be checked separately after parsing.

Explore the dry run

Guided walkthroughReady

1class Main {

2 static void showNumber(String text) {

3 try {

4 int value = Integer.parseInt(text);

5 System.out.println(value * 2);

6 } catch (NumberFormatException error) {

7 System.out.println("Invalid integer");

8 }

9 }

10 public static void main(String[] args) {

11 showNumber("12");

12 showNumber("abc");

13 }

14}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Pass valid numeric text.	`text = 12`	—
Parse successfully.	`value = 12`	—
Multiply and print.	`value = 12`	24
Pass invalid syntax.	`text = abc`	24
Parsing throws; control jumps to the handler.	`text = abc`	24
Print a recoverable error message.	`text = abc`	24 Invalid integer

Cost and common mistakes

O(n) parsing time for text length n, with bounded int storage.

Successful parsing does not prove the number meets application constraints.
Multiplying a large int can overflow; use Math.multiplyExact when overflow must be detected.

Try it yourself

Predict showNumber('-3').

Reveal answer and reasoning

It prints -6 because -3 is valid integer syntax.

Java methods and exception handling

Explanation

Follow the example

Cost and common mistakes

Try it yourself

Further reading